Top casting under pressure of rocket motor propellants



sept. 3o, 1969 s. GORDON ml. 3,470,273

TOP CASTING UNDER PRESSURE OF ROCKET MOTOR PROPELLANTS Filed May 17, 1967 5 Sheets-Sheet 1 2 v 25 I @6g 27- 2; @w

nw//W /se 9 A J' A v' I3 H32 f 2;:2 1;, "h, sk/ lgf fi, mf 6i FIG. l.

Sept. 30, 1969 y s. GoRDoN ETAL 3,470,273

TOP CASTING UNDER PRESSURE OF ROCKET MOTOR PROPELLANTS Filed May 17, 1967 5 Sheets-Sheet Sept. 30, 1969 s. GORDON Erm.. 3,470,273

TOP CASTING UNDER PRESSURE 0F ROCKET MOTOR PROPELLANTS Filed May 17, 1967 5 Sheets-Sheet 3 FIG'. 3. f fl 92--x\\\\ YY Sept. 30, 1969 s. GORDON ETAL 3,470,273

TOP CASTING UNDER` PRESSURE OF ROCKET MOTOR PROPELLANTS Filed May 17, 1967 5 Sheets-Sheet J.

Sept. 30, 1969 s. GoRDoN ETAL 3,470,273

TOP CASTING UNDER PRESSURE 0F ROCKET MOTOR PROPELLANTS Filed May 17, 1967 5 Sheets-Sheet i,

United States Patent O 66 Inf. Cl. cosh 21/02 U.S. Cl. 264--3 5 Claims ABSTRACT F THE DISCLOSURE Casting a solid propellant charge for a rocket motor is carried out by illing a mould cavity with casting powder, applying fluid pressure to a pressure plate to compress the casting powder, introducing a casting liquid into the compressed powder to displace air from interstices in the powder, maintaining pressure on the casting powder at a constant magnitude, and curing the propellant to a hard mass under the pressure applied to the propellant by the pressure plate.

Apparatus for carrying out the method includes a mold cavity, which may be a rocket casing, a pressure plate movable in the cavity, ducts through the pressure plate for the introduction of casting liquid, and a vent in the cavity wall for relieving air and excess liquid.

Background of the invention This invention relates to a method of casting, and head assemblies for casting machines for solid propellant grains for rocket motors.

Solid propellant grains for rocket motors are commonly produced by a casting process, particularly when their size makes the alternative extrusion processes diflcult to apply. The grain may be either cast in a mould and subsequently located in a rocket motor case or it may be cast directly into the motor case and bonded thereto during the casting and curing processes. This latter process is particularly useful when the outlets of the motor case are of smaller diameter than that of the body of the case. In such situations the propellant is said to be bottled in.

Casting is normally accomplished by loading a casting powder into a mould or motor case, displacing the air from the powder load by a casting liquid or solvent and then curing the resulting mass of powder and liquid to form the solid propellant.

Since the powder near one end face of the mass of casting powder is normally in contact with a surplus or reservoir of casting liquid or solvent during curing and the cured propellant consequently becomes over-rich in these, the cured propellant in this region tends to have a low density which gives rise to abnormal mechanical and ballistic characteristics. But it is of the greatest importance to achieve regular, uniform and completely reproducible ballistic, physical and mechanical properties in propellant grains, since these control the subsequent functioning and performance in rocket motors. Because of this, it has hitherto been necessary to cast the propellant in a length greater than its final desired length and then remove the region having abnormal properties by machining to Iesult in the desired length. However, this procedure is both hazardous, wasteful and time-consuming, and is doubly hazardous when applied to bottled in charges because of the proximity of the steel motor case during machining.

Furthermore, to satisfy the motor design requirements, it is frequently necessary to prepare propellant grains hav ing non-planar end faces.

ICC

It is also desirable that the density of propellant grains should not only be uniform but also that their overall density should approach as near as possible to of theoretical. To ensure this, it is necessary to pack the casting powder into the mould or motor case in such a way as to obtain as high a packing density as possible before the casting liquid is admitted. With nitrocellulose powders having good packing characteristics, 97-99% of the theoretical bulk density may be obtained although with modified powders, densities as low as 94% may be obtained.

The overall density of the final propellant is also affected by volume changes which occur during reaction of the casting powder and liquid and also during the subsequent cure. First, a shrinkage will occur, due to absorption of the casting liquid by the casting powder; this is followed by yswelling by the casting powder granules until they coalesce and gelatinise because of the chemical reaction between the liquid and the powder. Increase in temperature during curing produces a further increase in volume by thermal expansion. After curing shrinkage occurs on cooling down. In an ideal situation, the final volume would be very near the original starting volume of casting powder, plus casting liquid; however, in practice, this is not achieved since casting powder does not normally have its full theoretical density.

Thus it is desirable to densify and consolidate the propellant as much as possible in order to advance gelatinisation, promote homogeneity and uniformity, and develop optimum physical, ballistic and mechanical properties of the propellant.

Summary of the invention According, in one aspect of the invention a method of casting a solid propellant charge for a rocket motor comprises filling a mould cavity with a casting powder, compressing the powder under the influence of lluid pressure, displacing air from interstices between the granules of casting powder by the introduction of a casting liquid, and curing the propellant so formed to a hard mass, the fluid pressure being maintained at a substantially constant value from the iirst application of fluid pressure until the curing is completed.

In a further aspect of the invention a head assembly for a solid propellant rocket motor charge casting machine comprises a pressure member having a mould surface for the propellant during casting, the pressure member being movable relative to at least one other part of the assembly to vary the volume of a mould cavity and actuable by fluid pressure operable means to cause the pressure member to exert a substantially constant pressure upon casting powder in the cavity throughout the whole casting procedure of propellant when in use, and inlet means for delivering casting liquid or solvent to the mould cavity.

Compression of the propellant develops the bond system to a high degree and ensures uniformly good contact and good tenacious bonds over the propellant/case interface for a case-bonded motor.

The mould cavity may consist of the rocket motor case itself when a case-bonded motor is being formed or a separate mould for forming the configuration of a propellant grain alone. It is preferable, although not essential, to line the inside of a motor casing prior to the formation of a case-bonded motor with a suitable combustion inhibitor, e.g. cellulose acetate. The method of the invention may be used for the formation of various types of rocket propellant grains or case-bonded rocket motors, such as a single cigarette-burning propellant grain or the case-bonded grain, a motor having propellants case-bonded in two combustion chambers and a bi-propellant motor having one propellant grain cast radially inwardly of another propellant grain. Using the method and apparatus of the invention, a sound propellant grain of substantially 100% theoretical density may be obtained from casting powders of poorer quality than has previously been tolerable having densities as low as 92%.

The inlet means for delivering casting liquid to the mould cavity may open directly through a wall of the head assembly in the proximity of the top of the cavity so that the liquid can soak through the whole mass of casting powder and expel the air from the interstices of the casting powder, although in preferred embodiments, the casting liquid is introduced into a chamber behind t-he pressure member on the opposite side from the mould cavity and is then pumped into the cavity via channels passing through the pressure member. Passage of the casting liquid into the casting powder is assisted by pumping the liquid under pressure, preferably under a pressure ranging from to 100 p.s.i. of a gas inert to the casting powder and casting liquid, e.g. nitrogen, and this is effected in the preferred embodiments by pumping the inert gas under pressure into the chamber behind the pressure member.

Where the rocket motor consists of two or more separate propellant grains, such as a motor having propellants cast into separate chambers in the motor or a bi-propellant motor, it may be convenient to only partially cure the first-cast propellant grain to a sufficient degree to permit formers to be removed from the propellant without risking deformation of the grain configuration. Subsequent propellant grains may then be cast, while the first grain is suitably supported, if desired, on support formers preferably having draining ducts to allow for the escape of excess casting liquid in order to avoid softening and production of abnormalities in the partially cured-grain.

The invention also includes solid propellant charge made by the method or with the use of the head assembly according to the invention.

Brief description of the drawings In order that the present invention may be more readily understood, preferred embodiments thereof are described below by way of example only and with reference to the accompanying drawings, in which:

FIGURE 1 is a longitudinal sectional view of a machine for casting a single cigarette-burning charge;

FIGURE 2 is a longitudinal sectional view of a machine for casting propellant into the forward chamber of a casebonded motor having forward and aft propellant chambers;

FIGURE 3 is a longitudinal sectional view of a machine for casting propellant into the aft chamber of the motor shown in FIGURE 2;

FIGURE 4 is a longitudinal sectional view of a machine for effecting the rst stage casting of propellant in a bi-propellant motor; and

FIGURE 5 is a longitudinal sectional view of a machine for effecting the second stage casting of propellant in the bi-propellant motor shown in FIGURE 4.

Description of the preferred embodiments The casting machine illustrated in FIGURE l comprises a head assembly located over a cylindrical mould part 11, which with assembly 10 and end plate 12 defines a mould cavity. The surfaces of part 11 and plate 12 bordering the cavity are inhibited with cellulose acetate. An integral clamping flange 13 of a frusto-conical member 14 of the head assembly 10 serves in conjunction with a clamping member 15, which has a central aperture 16 shaped to house an annular end ring 17 of the mould part 11 and the end plate 12, for securely clamping, in a known manner, the head assembly 10, the mould part 11 and the end plate 12 together. The internal frustoconical surface of the member 14 is not inhibited.

The head assembly also comprises a pressure member consisting of a pressure plate 18 connected by a piston rod 20 to a piston 21 in a pneumatic control cylinder 22, the plate being movable in a cylindrical guide 19 integral with the frusto-conical member 14 in response to pneumatic pressure fluctuations communicated to the cylinder 22 via an inlet 23 in a head cap 24 attached to the cylinder 22. Excessive inward movement of the pressure plate 18 is prevented by arranging that a rear face 25 of the piston 21 should abut the end wall 26 of the cylinder 22 when the pressure plate 18 reaches the desired limit of its inward movement indicated by a dotted line A-A. Pressure in the space between the rear face 25 and the end wall 26 is normalised through a vent 27 during movement of the piston 21. A casting liquid inlet 28 discharges into a chamber 29 through an orifice 30. A nitrogen inlet 31 also discharges into the chamber 29 so that nitrogen pressure may be utilised for introducing casting liquid via channels 32 in the pressure plate 18 into the mould cavity. A casting liquid drain 33 is formed in the end plate 12.

The casting operation is commenced by introducing nitrocellulose casting powder in a known manner, conveniently by pneumatic entrainment, or alternatively by gravity flow, to achieve good powder loading density throughout the powder load, into the inhibited mould part 11 closed by the inhibited end plate 12, the head assembly 10 being released from the mould part 11, the end plate 12 and the clamping member 15 at this stage. The amount of casting powder introduced is regulated to a predetermined level which will produce a cured propellant grain of desired length and end configuration, when the face of the pressure plate 18 abutting the propellant mass lies in the plane defined by the line A-A. After securing the head assembly 10 to the mould part 11 and the end plate 12 by means of the clamping member 15, pressure is applied by the pressure plate 18 to the propellant powder by introducing a pressure fluid such .as air from a pressure source, through a supply line (not shown) and through the inlet 23 into the cylinder 22, thus moving the piston 21 and the pressure plate 18 to compress the casting powder in the mould cavity. The pressure of air in the cylinder 22 is selected from the range 10-1000 p.s.i. so that a predetermined pressure of p.s.i. is supplied by the pressure plate 18 for the compression of the powder. This pressure is maintained constant throughout subsequent stages of the casting operation by the provision of a relief valve in the supply line. The valve is arranged to admit air through the inlet 23 when the pressure on the powder is below 100 p.s.i. and to permit the egress of air when the pressure exceeds this value. Nitroglycerine casting liquid is introduced into the chamber 29 through the inlet 28 and nitrogen under a pressure ranging from 5-10 p.s.i. is simultaneously introduced through the inlet 31 so that the nitroglycerine casting liquid is entrained with a uniform flow rate under nitrogen pressure through the channels 32. A nitrogen pressure of up to 100 p.s.i. can be used.

Contraction of the nitrocellulose casting powder mass occurs during absorption of the nitroglycerine by the powder, the pressure of 100 p.s.i. exerted by the pressure plate -18 being maintained by successive additions of air through the inlet 23 corresponding to the progression of the shinkage. The travel of the pressure plate 18 is limited to project to a maximum distance of 1/s-inch past the line A-A in the direction of the propellant by the face 25 abutting the end wall 26. Swelling of the powder eventually results on account of the chemical reaction between the powder and the liquid so that the pressure plate is forced to retract and air is released through the inlet 23 and the valve to maintain the 100 p.s.i. pressure. The casting liquid is permitted to run out of the mould 11 through the liquid drain 33. During the swelling of the powder the granules tend to coalesce. Passage of the nitroglycerine casting liquid through the nitrocellulose is maintained until all air has been expelled from the interstices in the powder and carried away with the outowing nitroglycerine casting liquid. The flow of the nitroglycerine casting liquid is then terminated and the liquid drain 33 is sealed at the same time by means not shown. Curing is eiected by heating the propellant mass to a temperature `between 1GO-160 F., typically 150 F. for a period between 24-144 hours, typically 96 hours causing thermal expansion which is also accommodated by movement of the pressure plate. Pressure is maintained at 100 p.s.i. and eventually by the termination of curing and after cooling to ambient temperatures, the face of the pressure plate 18 contacting the propellant returns to the plane indicated by the line A-A to provide the desired end conguration and length of the cured propellant.

Examination of the cast propellant showed that air had been effectively excluded and that it had a uniform denstiy of substantially 100% of the theoretical density. The physical and ballistic properties were also satisfactory.

FIGURE 2 relates to the lirst stage casting of propellant for a case-bonded motor into a forward chamber 41 of a cylindrical steel rocket housing 40 divided into forward and aft chambers 41 and 42 -by an intermediate housing member 43. A head assembly 44 bolted by bolts 45 through its flange 46 to ilange 47 of a mounting ring 48 is located on the housing 40 by tting the upper end of the housing 40 into an annular housing recess 49 in the mounting ring 48. The mounting ring 48 is clamped to the housing 40 in a known manner by means of a clamping ange 50 integral therewith and a base plate 51 located over the other end of the housing 40 by an annular groove 52. A central core rod 53 tapering towards its upper end to correspond to the desired configuration of the central conduit is a sliding t in an inner former 54 and is located coaxially with the housing 40. The inner former 54 is attached by bolts 55 to a suspension plate 56 whose periphery is clamped between the anges 46, 47. A collar 57 mounted on the upper end of a tubular member 58 is centered on the axis of the housing 40 when the ange 50 of the mounting ring 48 is clamped to the base plate 51 on account of its upper frusto-conical surface 59 co-operating with a corresponding frusto-conical face of the housing member 43. Location of the core rod 53 axially in the housing 40 is assisted by a central bore 60 in the collar 57 through which a lower narrowed section 61 of the core rod 53 is passed.

A pneumatic control cylinder 64 formed in the head assembly 44 has a piston 65 movable axially therein under the inuence of pneumatic pressure applied through an air inlet 66 in a head cap 67 attached to the control cylinder 64. Air is supplied to the inlet 66 through a supply line having a Valve (not shown) which is arranged to admit air through the inlet 66 when the pressure exerted on the propellant powder is below 50 p.s.i. and to permit egress of air when the pressure exceeds this value. Movement of the piston 65 is transmitted through a piston rod 68 integral therewith and passing through a guide 69 in an end wall 70 at the other end of the cylinder 64 from the head cap I67 to a coupling member 71 passing through apertures 72 in the suspension plate 56 to bear upon a pressure member consisting of a pressure plate 73 which is located with a close sliding fit internally between the mounting ring 48 and the inner former 54 and is part of the head assembly. The same internal radius is provided on the mounting ring 48 as the housing 40 to permit the unimpeded movement of the pressure plate 73 within the two during casting. Dimensions of the piston rod 68, the coupling member 71 and the pressure plate 73 are such that a face 74 of the piston 65 is in abutment with the opposing face 75 of the end wall 70 when the casting operation is complete.

A liquid inlet 76 in the end wall 70 serves for the introduction of casting liquid which is directed through a tube 77 into the inside of the head assembly 44 situated between the end wall 70 and the pressure plate 73.

Nitrogen pressure from nitrogen introduced through a nitrogen inlet 78 is used to entrain the casting fluid through passages 79 in the pressure plate 73 into the housing 40. Ducts 80 in a gasket 81 serve to transfer excess casting liquid into an annular collecting channel 82 from which it is directed away through a liquid drain 83 and the tubular member 58 and finally through an aperture 84 in the base plate 51.

The inner former 54, the pressure plate 73 and the housing member 43 are proled so that the required ballistic prole results on the cured propellant. Normalisation of the pressure between the faces 74 and 75 is permitted during movement of the piston 65 by a vent 85.

In preparing for casting, the head assembly 44 and mounting ring 48 are not initially attached to the housing 40 but the core rod 53 is located in place. Nitrocellulose casting powder in a predetermined quantity required to produce the propellant for the forward chamber to the desired level is introduced into the housing 44 and the mounting ring 48 and head assembly 44 are placed on to the housing 40 and secured by means not shown by the flange 50 and the base plate 51. Compaction of the nitrocellulose powder is effected by introducing air through the inlet 66 to move the piston 65 inwardly so that a pressure of 25 p.s.i. is applied to the powder. This pressure is maintained continuously throughout the casting operation by means of the relief valve in the air supply line. When compaction has been accomplished nitroglycerine casting liquid is introduced through the inlet 76 and the tube 77 and nitrogen through the inlet 78 simultaneously under a pressure of 40 p.s.i. whereupon the nitroglycerine casting liquid is supplied under pressure through the passages 79 in the pressure plate 73 and is forced through the nitrocellulose powder, eventually flowing out through the ducts 80 and the liquid drain 83. Flow of the nitroglycerine is maintained at a uniform rate for such a time that substantially all air associated with the powder has been expelled. The liquid drain 83 is then sealed by means not shown and the flow of nitrogen and nitroglycerine is terminated. As the absorption of nitroglycerine progresses, a corresponding contraction of the powder occurs so that the valve in the air line introduces air through the inlet 66 in order to urge the pressure plate 73 inwardly to maintain the constant pressure exerted upon the powder. Reaction of the nitroglycerine with the powder in turn causes swelling of the powder until coalescence results, the constant pressure of 25 p.s.i. still being maintained. The temperature is then raised to 110 F. for a period of 24 to 48 hours to partially cure the propellant so that it is suciently solid to withstand movement and remain secure in the housing 40. Thermal expansion occurs as the tempertaure is raised and air is released through the valve in the air line to maintain the 25 p.s.i. pressure on the propellant. After completion of the partial curing, the propellant is permitted to cool while the constant pressure is maintained. When the partial curing has been effected, the face 74 of the piston 65 is in contact with the face 75 of the end Wall 70 and the pressure plate is positioned as indicated by the dotted lines B-B.

Upon completion of the partial curing, the head assembly 44 and the base plate 51 are removed from the housing 40 which is inverted into the position shown in FIG- URE 3 for the casting of propellant into the aft chamber 42, the core rod 53 being retained in position within the partially cured propellant in the forward chamber 41.

The casting of propellant into the aft chamber is now described with reference to FIGURE 3 in which like reference characters are used to refer to components common to FIGURES 2 and 3.

Before actually inverting the housing 40, an aluminium former proled to match the end profile of the propellant in the forward chamber 41 is placed inside the open end of the forward chamber and over the end of said propellant. A central bore 91 accommodates the end section of the core rod 53. The inverted housing 40 is supported upon a base plate 92 by locating its edge associated with the forward chamber 41 in an annular groove 93 therein, the end of the core rod 53 being firmly held in a cavity 94 in the base plate 92. Spring means 95 are located between the former 90 and the base plate 92 to urge the former 90 against the end of the propellant in the forward chamber with a pressure of 25 p.s.i. to ensure that the profile of the propellant is retained. Draining ducts (not shown) are located in the former 90 to drain away surplus nitroglycerine which would otherwise tend to soften the partially cured propellant.

A head assembly 96 is used which is identical with the head assembly 44 shown in FlGURE 2 apart from the configuration of the inner former which is designated by reference character 97 in FIGURE 3 and a casting liquid drain 98 which is also provided in the head assembly 96, there being no corresponding drain in the assembly 44. The remaining features of the head assembly will be understood by reference to the description of the head assembly 44 in respect of FIGURE 2. The head assembly 96 is securely attached to the housing 40 by clamping the flange 50 of the mounting ring 48 to the base plate 92.

A sheath member 99 co-axially enclosing the narrowed section 61 of the core rod 53 has an outer diameter equal to the diameter of the non-tapered section of the core rod S3 associated with the forward chamber propellant and traverses a central bore 100 in the inner former 97 in which it is firmly held. Vents 101 in the lower end of the sheath member 99 permit the passage of casting liquid through a clearance 102 between the sheath member 99 and the narrowed section 61 of the core rod 53 to an outlet duct 103 in a solid upper section of the sheath member 99 which communicates with the casting liquid drain 98.

Nitrocellulose casting powder is fed into the aft chamber 42 by pneumatic entrainment with the head assembly 96 removed from the housing 40, the amount of nitrocellulose casting powder supplied being a predetermined amount such that the pressure plate is positioned as indicated by the dotted lines C-C when the propellant has been cured. Compaction of the nitrocellulose powder is effected, after clamping the head assembly 96 to the housing 40 by means of the flange 50 and the base plate 92, by supplying air through the inlet 66 to the cylinder 64 so that a pressure of 50 p.s.i. is exerted on the powder by the pressure plate 73 and this is maintained throughout the process. Nitroglycerine casting liquid is introduced through the inlet 76 and the tube 77, and nitrogen through the inlet 78 under a pressure of 30 p.s.i., so that the nitroglycerine casting liquid is forced through the passages 79 in the pressure plate 73 and subsequently soaks through the nitrocellulose powder. The flow of the nitroglycerine casting liquid is maintained until substantially all of the air held in the interstices of the nitrocellulose powder has been expelled with the nitroglycerine casting liquid flow which leaves the aft chamber 42 by owing through the vents 101, the clearance 102, the outlet duct 103 and the liquid drain 98. Passage of the nitroglycerine casting liquid firstly causes contraction of the powder as the nitroglycerine is absorbed followed by swelling with coalescence of the powder. The 25 p.s.i. load on the powder is maintained by the valve in the air line connected to the inlet 66. When the expulsion of the air is complete, the liquid drain 98 is sealed off by means not shown with termination of the nitrogen and nitroglycerine casting liquid supplies and the propellant in the aft chamber 42 is cured simultaneously with the complete curing of the partially cured propellant in the forward chamber 41 by heating at a temperature of 110 F. for a period of from 96 to 120 hours. After removal from cure the head assembly 96 and the base plate 92 are detached by releasing the clamping between the flange 50 and the base plate 92, followed by removal of the core rod 53 and the sheath member 99 from the central conduit of the propellant CTI and removal of the former from the end of the forward chamber.

It was found that a case-bonded motor cast in this manner had properties similar to those found for the grain cast as described with reference to FIGURE l.

FIGURE 4 shows a head assembly 110 for casting a first stage propellant grain 111 in a mould part 112 which assembly is secured to the mould part 112 during casting by applying clamping means (not shown) between a clamping flange 113 and a base plate 114 when the head assembly has been located over the mould part 112 by inserting a rim 115 of the mould 112 in an annular depression 116 in the flange 113 and the mould part 112 has been located over the base plate 114 by supporting rings 117. A cylindrical core former 118 having a central rod 119 is located centrally in the mould part 112 to shape the inner cylindrical surface of the propellant grain 111. An enlarged end 120 of the rod 119 projects through an aperture 121 in the bottom of the mould part 112 and into a depression 122 `corresponding thereto in the base plate 114 to assist in locating the mould 112 on the base plate 114, Channels 123 in the core former 118 communicate via ducts 124 with a casting liquid drain 125.

The head assembly 110 comprises a piston 127 -movable axially in a pneumatic control cylinder 128 under the influence of pneumatic pressure applied through an air inlet 129 in a head cap 130 attached to the control cylinder 128. Air is supplied to the air inlet 129 through a supply line and a valve (not shown) which is arranged to maintain a constant pressure on the propellant 111 as described later by admitting air through the inlet 129 when the pressure is below that value and by permitting the egress of air when the pressure is above that value. 131 serves to normalise pressure between the piston 127 and passing through a guide 133 in an end wall 134 at the other end of the cylinder 110 from the end cap 130 transmits movement of the piston 127 to a coupling member 135 movable therewith to move a pressure -member 136 inwardly to compre-ss the propellant grain 111. A vent 131 serves to normalise pressure between the piston 127 and the end wall 134. The pressure member 136 is a close sliding fit between the core former 118 and the inner surface of the mould 112 and the wa-ll 137 of the head assembly 110 co-axial with the mould part 112.

A casting liquid inlet 138 through the control cylinder 128 and the end wall 134 serves for the supply of casting liquid via a tube 139 communicating therewith into the space within the wall 137. A nitrogen inlet 140 also communicates with this space within the wall 137 so that casting liquid may be entrained under nitrogen pressure through passages 136a in the pressure member 136 into the mould part 112.

For casting of the propellant grain 111, the mould part 112 having the core former 118 in position is set up on the base plate 114 by means of the supporting rings 117 and a predetermined quantity of nitrocellulose casting powder sufficient to fill the mould part 112 to the level of its rim when cured is pneumatically introduced into the space between the mould part 112 and the core former 118. The head assembly 110 is attached to the mould part 112 by clamping the flange 113 to the base plate 114. Air introduc/ed through the inlet 129 into the cylinder 128 moves the piston 127 and in its turn the pressure member 136 inwardly to exert a pressure of 125 p.s.i. on the nitrocellulose and to compact it. The constant pressure of 125 p.s.i. is continuously maintained through the casting process by means of the valve in the air supply line (not shown). After compaction of the powder, nitroglycerine casting liquid is introduced into the space within the wall 137 through the inlet 138 and the tube 139 being pressurized -by nitrogen gas at a pressure of l0 to 20 p.s.i. so that the nitroglycerine casting liquid is entrained through the passages 136a in the pressure member 136 to soak through the nitrocellulose in the mould part 112 and eventually flow out through the channels 123, ducts 124 and liquid drain 125. Passage of the nitroglycerine casting liquid is effected until all of the air trapped in the interstices in the nitrocellulose powder is removed when the liquid drain 125 is closed by means not shown and the flows of nitrogen -and nitroglycerine casting liquid are terminated. During passage of the nitroglycerine casting liquid absorption occurs by the nitrocellulose powder which is accompanied -by a corresponding contraction in Volume of the powder. Pressure is maintained at a constant value on the powder by the introduction of air into the lcontrol cylinder 128 so that the pressure member 136 is moved inwardly against the powder by an appropriate distance to maintain the constant pressure on the powder. Swelling of the powder eventually occurs on account of the reaction with the nitroglycerine casting liquid, the pressure still being maintained constant by the outow of air from the Ycylinder 128 through the valve in the air line permitting the pressure member 136 to move upwardly. Complete curing is eifected by heating the nitrocellulose/nitroglycerine propellant at a temperature of 100 to 110 F. for a period of 24 to 72 hours. The head assembly 110 is then removed from the mould part 112 by releasing the clamping between the flange 113 and the base plate 114. The core former 118 is carefully removed from the cured propellant grain 111.

Casting of the second stage propellant 145 is effected by means of the apparatus shown in FIGURE 5 in which components substantially identical with those used in the apparatus shown in FIGURE 4 have been allocated like reference characters, details of which may be understood from the description with reference to FIGURE 4.

A cylindrical core former 146 of smaller diameter than the core former 118 located in the mould part 112 containing the cured rst stage propellant grain 111 has a central rod 147. An enlarged end 148 of the rod 147 projects through the aperture 121 in the bottom of the mould part 112 and into the depression 122 corresp-ondmg thereto in the base plate 114 to assist the supporting rings 117 in locating the mould part 112 on the base plate 114. Channels 149 in the core former 146 communicate via ducts 150 with the casting liquid drain 125.

The head assembly 110 is the same as that shown in FIGURE 4 except that a pressure member -151 replaces member 136 and an annulus 152 is secured by means not shown to the wall 137 over the first stage propellant grain 111. The annulus 152 has an inner `cylindrical surface of the same radius of curvature as the inner surface of the first stage propellent and serves to guide the movement of the pressure member 151 in conjunction with the core former 146, Overlling of casting powder -above the level of the iirst stage propellant can be conveniently effected on account of the provision of the -annulus 152. The pressure member 151 has passages 151a to permit transfer of casting liquid from the space within the wall 137 to the other side of the pressure member 151 =for soaking the casting powder with casting liquid.

Casting of the second stage propellant is effected by pneumatically lilling the space between the first stage propellant and the core former 146 with nitrocellulose casting powder, the head assembly 110 being detached from the mould at this stage. The amount of nitrocellulose used is a predetermined amount which, when cured, 'will have a surface at the open end of the mould level with the surface of the rst stage propellant. Such a predetermined quantity necessitates over-filling above the final level so that the extension piece 152 serves to confine the nitrocellulose powder.

The head assembly 110 is replaced and clamped by the ange 113 to the base plate 114, compaction of the nitrocellulose powder then being effected by introducing air at a pressure of 40 p.s.i. through the inlet 129 into the cylinder 128 to move the piston 127 and in turn the pressure member 151 inwardly. The pressure of 40 p.s.i. is maintained constant throughout the whole casting process by means of the valve in the air supply (not shown) in order to exert a constant pressure on the propellant for the whole of the casting process. Nitroglycerine casting liquid is introduced into the space within the wall 137 through the inlet 138 and the tube 139 simultaneously by the application of nitrogen at a pressure of 10 to 20 p.s.i. The pressure of the nitrogen serves to force the ntroglycerine casting liquid through the passages 151:1 in the pressure member 151 and through the casting powder located between the first stage propellant grain 111 and the core former 146. The nitroglycerine casting liquid eventually flows out of the mould 112 via the chan nels 149, the ducts 150 and the liquid drain 125. Passage of the nitroglycerine casting liquid is continued until all the air in the interstices of the nitrocellulose powder has been expelled in the nitroglycerine casting liquid stream through the liquid drain 125. The liquid drain is then sealed by means not shown and the ows of nitrogen and nitroglycerine casting liquid terminated. Shrinkage in the volume of the nitrocellulose powder is caused by the absorption of nitroglycerine casting liquid, the pressure exerted on the powder being kept constant by the valve in the air line to the inlet 129 regulating the pressure of air in the cylinder 128. On account of the absorption of nitroglycerine, the nitrocellulose powder eventually swells up, the air pressure still being maintained at the constant value of 40 p.s.i. Curing by heating at a temperature of 110 F. for a period of 92 to 120 hours is nally effected while still maintaining the constant pressure on the propellant.

Propellant cast into a bi-propellant motor by this procedure was found to have properties comparable to those of the grain cast in accordance with the procedure described with reference to FIGURE l.

We claim:

1. A method of casting a solid propellant charge for a rocket motor comprising lling a mould cavity with granules of a casting powder, contacting the granules of casting powder with a pressure plate, applying Huid pressure to the pressure plate to compress the granules to occupy a reduced volume of the mould cavity, introducing a casting liquid through the pressure plate into the compressed granules under gas pressure to displace air from interstices between the granules, maintaining the uid pressure on the pressure plate at a constant magnitude, and curing the propellant so formed to a hard mass under the pressure applied to the propellant by the pressure plate.

2. A method according to claim 1 wherein pressure is applied to the casting liquid by a gas inert to the casting powder and the casting liquid and under a pressure of 5- p.s.i.

3. A method according to claim 2 wherein the inert gas is wherein the powder is compressed using compressed air at between 10 and 1000 p.s.i.

4. A method according to claim 1 wherein the powder is compressed by compressed air at between 10 and 1000 p.s.i.

5. A method according to claim 1 wherein the propellant is cured at 100-160 F. for 24 to 144 hours.

References Cited UNITED STATES PATENTS 3,027,597 4/ 1962 McCurdy 264-3 3,202,730 8/ 1965 Gordon et al. 264-3 3,205,286 9/ 1965 Black 264--3 3,222,433 12/ 1965 Makay 264--3 CARL D. QUARFORTH, Primary Examiner MICHAEL I. MCGREAL, Assistant Examiner U.S. Cl. X.R. 

